1. Field of the Invention
This invention relates to optical coherence tomography. More particularly, it relates to a method and apparatus for improving optical coherence tomography devices used for detecting scattered optical radiation from internal structures in scattering media by reducing speckle due to multiply scattered light and without any loss of resolution.
2. Description of Related Art
Optical coherence tomography (OCT) is useful for imaging through highly scattering media. OCT allows for non-invasive, non-destructive cross-sectional optical imaging of scattering media with high spatial resolution and high sensitivity. OCT is related to white light interferomtery (WLI) and low coherence domain reflectometry (LCDR). WLI is used for one or two-dimensional profiling of surfaces. LCDR is used for one-dimensional depth probing of materials. OCT is used for two or three-dimensional internal probing of both scattering and non-scattering materials. The subject of the present invention pertains to application of OCT for study of scattering materials.
By way of general background, when a low coherence light source beam is directed into a scattering medium and reflects from in-homogeneities or internal structure, a speckle pattern is observed. The speckle pattern results from interference of different components of electromagnetic radiation which originate from a coherent source but which follow different paths in reflecting or scattering to the detector. At some points, the total field reflecting from the material will add constructively and be bright, while at other points the total field will add destructively and be dark. The signal detected in a given measurement has two components. One, which we will term the direct signal, arises from light that propagates without scattering to the region of interest(ROI), that is the region of the sample that is to be imaged, and is scattered backward from the ROI to the detector. The other component, the coherent MSL, arises from light that is multiply scattered over many different paths, all of which have the same optical path length as that of the direct signal. Although the direct signal arises only from the ROI, the coherent MSL arises from a much larger volume of the sample. The two signals combine coherently to form a speckle pattern, and the individual contributions of the direct signal and the coherent MSL are indistinguishable in usual measurements.
Various methods and apparatuses for obtaining tomographic information have heretofore been proposed wherein tomographic information, such as a tomographic image, of a medium having light scattering properties, is obtained such that the medium may not be invaded. With the OCT, a low coherence light beam is split into a light beam, which is to be irradiated to a medium, and a reference light beam, and a Michelson type of interferometer is constituted by the two split light beams. Optical heterodyne detection is carried out on the interference light beam, which is obtained from the interferometer, and the intensity of a light beam scattered backwardly from the medium is thereby determined. From the intensity of the backward scattered light beam, the tomographic information at the surface of the medium or an internal portion in the medium is obtained. Tomographic information at an arbitrary internal portion in the medium can be obtained by scanning the position of the ROI in the x-y plane and varying the optical path length of the reference light to control the z position for the depth into the medium.
It is well known in the prior art how to incorporate low coherence optical interferometers into various OCT apparatus to study scattering media. In a typical prior art optical fiber embodiment of the OCT apparatus, a low coherence radiation source and a photodetector are coupled to two input ends of a 3 dB coupler. The beams of radiation transmitted from two output ends of a 3 dB coupler are transmitted to a sample medium to be tested and a reference medium, respectively. The beams from the output ends are reflected from the sample medium and the reference medium, respectively; combined by the a 3 dB coupler; and transmitted to the photodetector.
The inventors have identified the speckled appearance of an image to be an important issue concerning typical OCT measurements. The problem of speckle is especially serious in the detection of internal structures, for which the spatial scale of the speckle is comparable with that of defects in the material, such as for example, subsurface cracks in ceramics.
Since OCT relies on coherent detection, the signal usually appears in the form of the speckle. While OCT discriminates against incoherent multiply scattered light (MSL), it fails to discriminate coherent MSL from light from a region of interest (ROI) in a sample. The coherent MSL that reaches a detector within the same coherence length as light from a ROI will be detected by OCT and will contribute to the detected speckle. It is generally recognized that some form of speckle averaging can be used for speckle reduction. Xiang et al., Proc. SPIE 3196, 79 (1997); and Schmidt, Phys. Med. Biol. 42, 1427 (1997), have used an array of four detectors to average speckle patterns.
The inventors have also recognized that speckle reduction in OCT is possible when the scattered signal light is detected and averaged in more than one direction. This can be accomplished with either a finite number of detectors in parallel, or by sequential angle averaging. Although the above techniques will reduce speckle from a ROI as well as from MSL, the penalty paid for the speckle averaging is the loss of optical resolution and also slower image acquisition. For example, in order to reduce the speckle by a factor of three, a total of nine images need to be collected with the optical resolution also deteriorated by a factor of three.
These techniques and the images produced by them are illustrated in FIGS. 1-4. Referring now to FIG. 1, there is shown a schematic of a low-coherence fiber interferometer based system 100 to detect subsurface defects in a sample 112. Input signal beam from a low coherence light source 102 is reflected by mirror 104, mirror 108 and is made to focus on the sample 112 via lens 110. The angle of incidence of incident light beam that is made to focus on the sample 112 is changed by translating the input signal beam in front of the lens 110 using a right angle prism 106. Translation of the prism 106 along its hypotenuse translates the input signal beam. Light scattered from the sample 112 is reflected back towards the incidence direction and completely repeats the path of the incident beam to be detected by detector 114. Scattered light is averaged over different angles of incidence of the signal beam where the signal is detected in the exact backscattered direction. Although the apparatus shown in FIG. 1 reduces speckle from a ROI as well as from coherent MSL, the penalty paid for speckle averaging is the loss of optical resolution and slower image acquisition. The loss of resolution occurs because angle averaging requires that the input beam not fill the lens. Since the return beam retraces the path of the incident beam, the resolution is determined by the relatively small diameter of the beam at the lens.
FIG. 2 is an OCT scan taken at a depth of 80 μm, as measured in air and at different angles of incidence using the apparatus shown in FIG. 1. The sample used for taking the OCT scan is a flat piece of Si3N4 with a surface-penetrating Hertzian circular crack. Three representative X-Y OCT scans taken at a depth of 80 μm, as measured in air and at different angles of incidence, are shown in FIG. 2. The scan size is approximately 2.5 mm by 2.5 mm. The signal to noise ratio (SNR) was measured to be ˜5. The images are observed to have a characteristic speckled appearance. The speckled appearance of the images represents a significant difficulty in crack detection.
Averaged OCT images with and without changing the incidence angles were also compared. As the speckle pattern does not change from shot to shot, the averaging of the images at one incident angle merely reduces the noise without affecting the speckle. The result of 11 combined OCT scans at the same angle is shown in FIG. 3. The SNR was measured to be ˜8, and the crack is still undetectable.
FIG. 4 shows an image illustrating the effect of speckle reduction due to averaging 11 images at different angles of incidence. The SNR was measured to be ˜20. The effect of speckle reduction is directly confirmed by the clearly visible subsurface circular crack.